Thank you for contacting us! Our Steam store page does not have a demo yet. We wanted to get feedback from the Itch community and also add some extra content before uploading a demo to Steam.
But both versions will be exactly the same, only the Steam version will have Steam-specific integrations added (eg. achievements, cloud saving).
Same goes for the two versions available on Itch (web and downloadable). The downloadable version runs a bit better and has minor accessibility improvements.